John Sayers' Recording Studio Design Forum Archive

Hi there!

After long time of planning, some aspects of our new studio shall be nailed, so I want to make the final calculations for the slat absorber on the side walls of our control room. After measuring our room with MLSSA, I know where the problematic frequencies really are.

Using this really great spreadsheet, I got quite weired results:

http://www.whealy.com/acoustics/Porous.html

Now to my question: What`s your experience with slat absorbers? Do I have a chance of getting decent absorption at 150Hz??
Or is this frequency range too low for a slat absorber?

When you calculate the resonance with the standard excel file (which John recommends), you can easily get a resonant frequency of 150Hz.
(Depth: 45mm, Slat width: 150mm, Slat thickness: 19mm, Slot width: 1mm --> 156Hz).
Is this absorber going to be effective? With 1mm slots? Or is this a case where the formula doesn`t represent the physical behaviour?

In which frequency ranges do your slat absorbers work?
Should I rather apply a bigger helmholtz resonator for those frequencies?

Thank you for your help!

regards.
Schroeder

Reading your post, a question comes immediately to mind. IF you are interested in absorption at 150 Hz would not rockwool at 100mm thickness be adequate? If you are concerned with initial reflections then 200 mm will do. Much less work than building a tuned absorber.

At the dpeth that you indiciated (19 mm) you can not get down low enough without very small slots.

Andre

edit 06.01.26 changed changed "buisling" to an english word "building."

The control room follows the RFZ design with reflecting surfaces in the front.
The backwall of the control room is absorptive, with mineral wool (similar to 703) und Rockwool from 100 to 300mm.
The ceiling has also a section with mineral wool (no splayed ceiling possible -> RFZ) and a section with slats (50mm depth)
I was also referring to John`s designs with slat absorbers on the side walls.

The main reason was not to deaden the very small room (12,6qm) and to use the little amount of space at a max.
I thought I could increase the amount of diffusion in the highs and absorb the low mids/mids with the slat absorbers.
My goal was to archive a really acurate room with not too much absorption of the highs.
I think there will be problems with 150/200Hz, perhaps some muddy/tinny sound from around 500Hz or so, as always in such small rooms. I just tried to battle those frequencies without killing the highs too.
What do you think? Besides the extra amount of time...
What frequencies are best for an slat absorber with 45mm depth and 19mm slats? 200Hz and above? 300Hz and above? Or even higher? I don`t know. I don`t have any practical experience...

You say there`s no chance of reaching 150Hz effectively... ok that`s a statement.
What about using the upper part of the slats for the higher frequencies and the lower part near the floor for an helmholtz box with same depth but only one bigger slot (just have to calculate it in a correct way with correction of the opening, etc.)
That should work fine.

regards,
Schroeder

Thanks for the quick reply. If I am reading your last post correctly, you are designing the room using ideas that you have seen here and other sites without doing the acoustic calculations. The danger is that by using tuned absorbers at one range, you usually leaving a deficiency of absorption at other frequencies. That deficiency then has to addressed with other tuned absorbers. If that is the path (multiple tuned systems) that you taking, fine. It wasn't clear from you posts that you have done the calculations for that method to be effective.

This quote is misleading

You say there`s no chance of reaching 150Hz effectively... ok that`s a statement

Because it ignores the full statement which was

At the dpeth that you indiciated (19 mm) you can not get down low enough without very small slots.

The significant part being "at the depth you indicated (19 mm)".

Good luck and keep the questions coming unitl you are satisfied with all the answers you recieve and the answers you give to questions asked about your design!

Andre

Ok, the point why I was asking is that I have absolute no (own) experience with slat absorbers.
I`ve built a home studio a few years ago which already had an angled ceiling (acoustic panels), low frequency absorption which we archived with a combination of panel absorber and helmholtz, etc.
It sounded quite good in that very small room, although we hadn`t really calculated the absorbers in detail. Just did some coarse cowboy calculations.

In our new studio, which is going to be our working enviroment for professional productions (no studio to rent, just our own projects), it`s quite a different thing. It will have lasted about a year to build the whole thing (we will be finished next June I think ) and we are quite accurate in planning, calculating and constructing.
We already had two days of measuring with MLSSA in different states of the construction, so we could compare them. We already did all the calculations (modes, T60 Sabine, T60 Eyring), etc. You see, were not afraid of calculating and planning...

But there is one problem: We don`t have practical experience with absorbers when it comes to designing a room to be very very acurate.

When you look at some of the standards that have to comply with when you want to call your studio "pro" (ITU, AES, etc.) you have to make your studio better than good... (for example: A german organisation that consists of the IRT, SRT and VDT; they provide lots of "standards" for recording studios, broadcast, etc.; have published a paper that says your control room frequency response has to be within +/- 3dB nearly over the whole range!!! (only a slight roll off in the highs))
We just wanted to archive something near this recommendation.
We applied heavy bass absorption, an decent RFZ with splayed walls, etc.
The only thing we are not really sure about is how much absorption one needs in the low mids?? I think this frequency range is very problematic because of its boomy character...
When I first saw how often John uses slat absorbers in recording and control rooms, I thougth this kind of absorber could be cool. Good lookin, not too much absorption in the highs and an additional amount of diffusion! That`s great, isn`t it? At least better than applying open rockwool with cloth or foil and cloth everywhere.
Now the problem is how to get an idea of the frequency range where these absorber are REALLY effective. It`s not enough knowing the resonant frequency. These absorber seem to function between some borders, given by physics. As you say, they are not really effective at 150Hz, although the resonant frequency WOULD be at 150Hz!
Do you have any DON`t DOs? Something like: Don`t design them with a slot width of 1mm, that`s too small, better go with 5mm and above?...

About getting a lack at discrete frequencies by absorbing with a tuned absorber: You can vary the slot width to get the absorber to work in a certain range. If you apply enough dampening (rockwool or similar), you broaden the response too, BUT without having lack of high frequencies as with open Rockwool, 703, etc.

Thoughts?

Cheers,
Schroeder

Thanks for the additional information. It helps alot. I also read Chris's reply in this thread on Studiotips. Unfortunately what I hoped to avoid is starting: slightly diferent directions on the same general question on two different sites.

From what I have read on both threads you have identified the 125 Hz range as a concern. you are thinking about choosing a slat or a Helmhotz absorber. The two are the same thing!

With regards to the effective lf absorption, hav a look at fig. 1 and 2 in BBC RD 1992-11 to the the actual measured absorption of absorbent material.

Do you have any DON`t DOs? Something like: Don`t design them with a slot width of 1mm, that`s too small, better go with 5mm and above?...

Not so much a do or do not as "be aware of." Slat absorbers with a percent open area greater than 3% act more like the absobent material behind for low frequencies and a reflector at frequencies with wavelenghts shorter than the width of the slats. There is no significant effect of lowering the peak absorption frequency.

I'm familiar with most control room/listening room standards and admire your decision to the standards and design to those recommendations. If you look back at the two figs I referenced above you will see the significant effect of increasing depth of material id to increase the lowest effective frequency of absorption, especially for down 125 Hz for incident sound. Something overlooked so often by studio designers!

I hope you get some benefit from this in helping achieve a great sounding room!

Andre

Thanks Andre!

Another step forward for me! Sometimes one needs to be pushed in the right direction...
My first thoughts were not quite realistic.
There was little depth at the sidewall, so I tried to find an absorber that could absorb bass with little depth...

Great BBC article! Have already read some of them concerning different topics. Always very practical!

About the 3% open area and its relation to different frequencies:
That`s a hint that helped me alot to get thngs in my head into the right order again. Great!

I`m going to write about our studio designing to finish my degree.
Do you have a link to an article, essay or something like that in www which I could quote (just concerning this 3% / wave length thing)?? Haven`t read that anywhere until now and I`ve read a lot...

Thank you very much for your help!

Schroeder

Do you have a link to an article, essay or something like that in www which I could quote (just concerning this 3% / wave length thing)?? Haven`t read that anywhere until now and I`ve read a lot...

Unfortunately no. It is something that is fairly obvious if you study graphs of Helmholtz absorbers with varying percent open area. Unfortunately, again, I can't recall any online sources. I do recall that it showed up quite well in a report by (you guessed it) BBC several years ago, but I don't recall the title.

I`m going to write about our studio designing to finish my degree

Great!. Many people would enjoy reading it and learning from it. If you want, you could post it here for us to enjoy!

Andre